1. Field of the Invention
This invention relates to a rolling method and a rolling apparatus capable of producing defect-free strips in any desired order of strips regardless of their widths to prevent irregular wear of work rolls.
2. Description of the Prior Art
With hitherto used tandem rolling mills, materials to be rolled were usually supplied into a rolling line in the order of the material decreasing their widths from the maximum to the minimum in order to avoid irregular wear on work rolls. Such a rolling effected in the order of the width of materials has been unavoidable.
The term "strip" as used herein is intended to include metal plates, such as mild steel strips, steel plates thicker than usual steel strips, nonferrous metal plates and the like.
This fact will be described in connection with cold rolling with reference to FIGS. 1 and 2 schematically illustrating the wear of work rolls. FIG. 1 shows a profile of a work roll of a No. 4 mill stand which has been used for rolling 300 tons of strips having a width of 775 mm. Local wear occurs apparently in the roll at locations corresponding to edges of the strips. FIG. 2 illustrates similarly a profile of a work roll of a No. 2 mill stand which has been used for rolling 850 tons of strips having a width of 775 mm. Wear occurs susbstantially uniformly over an area corresponding to the width of the strips. Initial roll curves of these work rolls were a sine curve of 2.5/100.
As can be seen from the above illustration, when strips of the same width usually more than 100 tons have been rolled, irregular wear has occurred on the work rolls over full widths of the strips or at the locations corresponding to the edges of the strips. If the work rolls with such irregular wear are used to roll further strips wider than the above strips, the unevenness of the irregular wear are directly transferred onto the wider strips to cause surface defects of the strips, resulting in reduced yield rate of the strips. In order to avoid this, therefore, it is generally obliged to set a rolling schedule in which strips are rolled in the order decreasing the widths from the maximum to the minimum. In contrast herewith, if strips are rolled in an order increasing the widths from narrower to wider strips, it is naturally needed to exchange work rolls to avoid the above defects.
Moreover, if strips thinner than 0.2 mm are rolled after a great amount of strips having the same widths have been rolled, edge defects as shown in FIG. 3a and edge cracks as shown in FIG. 4 would occur, even if all the widths of the strips are equal, resulting from the damage of the strip edges caused by local wear already produced as shown in FIG. 1. (FIG. 3b illustrates a normal strip edge.) These defects would also decrease marketable values of the products and yield rate, and in extreme cases, result often in breaking down of the strips.
Moreover, there is irregular wear of work rolls other than those above described as shown in FIG. 5 illustrating a profile of the work roll. This irregular wear is probably caused by the fact that extraordinary protrusions longitudinally continuously extending and called "high spots" often occur on the strips in hot rolling (as shown by arrows in FIG. 6), and the extraordinary protrusions would in turn damage work rolls in cold rolling. Such irregular wear causes further defects on strip coils called "built-up" as shown by arrows in FIG. 7. The defects are inadmissible for the strips as marketable products.